The invention relates to a system for transmitting digital information comprising a coding arrangement, a transmitting medium such as a record carrier, and a decoding arrangement. In such a system the digital information is received in groups of input words which are converted, in the coding arrangements, into code words representing the input words. Each code word corresponds to an input word, and is arranged for application to the transmission medium, the code words being applied to the decoding arrangement via the transmission medium and then converted back into digital information in the decoding arrangement.
The invention also relates to a coding and decoding arrangement and a record carrier for use in such a system.
In several applications, for example in systems where the information is optically recorded on the medium and optically read therefrom, such as the system described in non-prepublished Netherlands Patent application No. 8000121, corresponding to U.S. application Ser. No. 124,392, filed Mar. 26, 1980, now abandoned, the description of which is included in the description of the FIGS. 1-13, a number of considerations are of importance for the selection of the group of code words. Thus, for low frequencies, the random power spectrum of the code words must preferably contain comparatively little signal power in order to allow the addition of low-frequency servo signals and at least the continuous portion of the power spectrum must have at least one zero point in order to allow the addition of a clock signal. In addition, a number of parameters must be optimized, such as the information density. In view of the power of the laser, in a system having an optical record carrier, the information density must be related to the number of bits to be burned into the record carrier with the laser.
From the many known coding systems it appears, as described in the above-mentioned application, that the "quadphase" coding is very suitable for use in such systems. Quadphase coding is described in an article by U. Appel and K. Trondle: "Zusammenstellung und Gruppierung verschiedener Codes fur die Ubertragung digitaler Signale", published in the Nachrichtentechnishe Zeitschrift, Vol. 1, 1970, pages 11-16, FIG. 7 in particular. A quadphase-coding signal is obtained by dividing the original binary data signal into groups of two bits, designated dibits, and by placing the first and the second bit, respectively, of the dibit in a first and a second half bit interval, respectively, of the coded word which consists of two bit intervals, and placing the inverted value of the first and the second bit, respectively, of the dibit in the third and fourth half bit interval, respectively, of the coded word.